Color grading/matching strips in the VSE
There are 2 ways to do this:


You could stack the strips above each other and make a new preview window for each strip. At the bottom of the preview window is a channel number, simply set this for the strip you want to see.


Or you can make 2 preview windows and turn on the Ghost offset function in the preview window. Simply turn on the ghost button and enter the frame offset so the black line lands on the shot you want to compare. Then toggle down the Overlay menu next to the offset and select "reference". The other preview window should show "Current", this will display whatever the green play indicator is stopped on.